A generator is a device which is capable of adjusting a voltage (such as a high voltage) by a specific value. The generator may simultaneously also be implemented as a voltage source (for example, as a high-voltage source). In other words, in this case the generator provides both the voltage (such as the high voltage) and also a possibility of adjusting this voltage by a specific value.
Particle beam apparatuses, such as electron beam apparatuses, have been used for some time for studying samples. In particular, scanning electron microscopes and transmission electron microscopes are known.
In a transmission electron microscope, electrons of an electron beam, which are generated using a beam generator, are directed onto a sample to be studied. A part of the electrons of the electron beam is scattered in the sample. Non-scattered electrons and the scattered electrons are detected and used either to generate images of the sample or to generate diffraction images of the sample.
Scanning electron microscopes are used to study surfaces of objects (samples). For this purpose, in a scanning electron microscope, an electron beam (also referred to hereafter as a primary electron beam) is generated using a beam generator and focused by an objective lens onto an object to be studied. The primary electron beam is scanned over the surface of the object to be studied using a deflection apparatus. The electrons of the primary electron beam interact with the object. As a result of the interaction, electrons are emitted from the object in particular (so-called secondary electrons) or electrons of the primary electron beam are backscattered (so-called backscattered electrons). The backscattered electrons have an energy in the range of 50 eV up to the energy of the electrons of the primary electron beam at the object, while the secondary electrons have an energy of less than 50 eV. Secondary and backscattered electrons form the subsequent so-called secondary beam and are detected using a detector. The detector signal thus generated is used for image generation.
In both previously described particle beam apparatuses, the electrons of the primary electron beam are accelerated to a specific energy. For this purpose, the beam generator is kept at a high voltage, for example, in the range of 0.02 kV to 30 kV for a scanning electron microscope and 20 kV to 200 kV for a transmission electron microscope. In order to achieve a desired resolution, setting the high voltage to a specific value is known. Furthermore, in order to achieve a good resolution in the final images provided by the particle beam apparatuses, it is desirable to keep the voltage applied to the beam generator as stable as possible.
A high-voltage DC generator for a particle beam apparatus is known which has an AC voltage source, which is set by an amplitude regulator via a desired target value of the high voltage and whose output voltage is supplied to a step-up transformer. The step-up transformer steps up the AC voltage. The output voltage of the step-up transformer is in turn supplied to a Cockroft-Walton generator, which multiplies the output voltage of the step-up transformer. The high voltage resulting in this way is smoothed via a filter or multiple filters made of resistors and capacitors and supplied via a measuring resistor to the amplitude regulator. Fluctuations of the smoothed high voltage are detected via a capacitive divider, which includes a first capacitor and a second capacitor, and supplied to an amplifier. The amplifier provides an output signal, which is supplied to the amplitude regulator and acts in counterphase to the fluctuations of the smoothed high voltage. In this way, the fluctuations of the smoothed high voltage are additionally damped.
The system described above has the disadvantage that the amplitude regulator tends toward oscillations. Considerations have shown that limits are thus set on the stabilization of the high voltage. Therefore, not every gain is possible and the maximum achievable stability of the high voltage is thus narrowly limited.
Reference is made, for example, to DE 44 33 531 A1 and DE 44 33 524 A1 in regard to the prior art.
It is therefore desirable to provide a system in which basically any desired high voltage may be stabilized.